The art of blow molding objects from polymeric resins is known. Blow molding is a process for making hollow parts such as, for example, bottles and beverage containers from resins such as, for example, polyethylene terephthalate (PET), and is described in several literature articles. Some examples are Kirk-Othmer Encyclopedia of Chemical Technology, Third Ed., Vol. 18, 198, (1982); Encyclopedia of Polymer Science and Engineering, Second Ed., Vol. 2, Wiley-Interscience Pub., John Wiley & Sons, New York, 447 (1985); Concise Encyclopedia of Polymer Science and Engineering, J. I. Kroschwitz, ed., Wiley-Interscience Pub., 90 (1990); Molding of Plastics, N. M. Bikales ed., Wiley-Interscience Pub., 89 (1971); and Handbook of Plastic Materials and Technology, I. I. Rubin, ed., Wiley-Interscience Pub., 1063 (1990).
In a typical process for blow molding hollow articles from thermoplastic resins, a molten tube or cylinder ("parison") of resin is extruded from a die above an open mold. The parison hangs from the die on its own support, while the mold is closed around it, and the bottom of the parison is pinched together by the mold. Air or a similar gas under pressure is fed through the die into the parison, which expands to fill the mold, and the part is cooled as it is held under internal air pressure, and then released. The process requires that the resin exhibit sufficiently high melt strength, typically high melt viscosity, to stay in shape and retain integrity during the low shear conditions on the parison, but good flow during the high shear blowing process.
Generally, thermoplastic resins such as, for example, polyethylene (PET), are commonly used in blow molding hollow articles. Among engineering resins, few are suitable for this purpose, due to their generally low melt viscosity. Because of this low melt viscosity, the parison does not have adequate strength to support itself from the die. The problems encountered in the blow molding of engineering resins are discussed by J. M. Daly and K. F. Wissbrun, Melt Rheology and its Role in Plastics Processing-Theory and Applications, Van Nostrand Reinhold Pub., New York, 522-523 (1990).
A class of engineering resins that are of much interest in this regard is the liquid crystal polymers (LCPs). LCPs have enjoyed commercial success due to their excellent strength and high temperature stability. Several of them are commercially available such as, for example, the Vectra.RTM. resin from Hoechst Celanese Corporation, Engineering Plastics Division, Chatham, N.J. LCPs also suffer from the same disadvantage of low melt viscosity for blow molding. Because of their high potential for specialty applications, there has been much interest in improving their melt viscosity to make them suitable for blow molding.
One method that has been tried in the past is to use external filler matter such as glass, fiber and the like, to increase the melt viscosity of LCPs. Pending application, Ser. No. 07/967,896, filed Oct. 28, 1992 discloses filled LCPs suitable for blow molding applications. While this approach helps to some extent, it limits the applications where such resins could be used. For example, blow molded articles from such filled resins may contain certain porosity in their walls that may make them unsuitable for applications such as gas containment. It would be preferable to blow mold such articles from unfilled, homogeneous LCP resins, if their melt viscosities can be sufficiently increased.
Yet another approach that has been tried is to heat treat LCPs suitably to increase their melt viscosity and melt strength. Pending application Ser. No. 07/979,102, filed Nov. 20, 1992, discloses such a process. Such an approach involves the extra step of heat treatment with attendant possible cost increases. It would be preferable if the LCP resin has a composition that makes it suitable for blow molding directly, without having to resort to additional steps, or added filler matter.